Corrected balance_4dvar.m script

[arango]

Corrected matlab script balance_4dvar.m used to compute the balanced/unbalanced contribution for 4DVar error covariance matrix standard deviation scale. We need to use the total T and S increments when computing the density increment delta_rho, as reported in Weaver et al. (2005) balance operator in Eq (18) and (22).

Many thanks to Andy Moore for reporting this problem.